How to Choose a Power Bank: Wh vs mAh, Wattage, and What Actually Matters

Most power bank buying advice focuses on the wrong metric. Rated mAh is the spec that gets the most marketing attention and the least useful for making purchase decisions. Wh and output wattage are what actually determine whether a power bank does the job you need it to do.

Why mAh Is a Misleading Spec

mAh (milliamp-hours) measures charge at a specific voltage. The problem: manufacturers can choose different voltages to inflate the apparent mAh number.

A battery pack with 20,000 mAh at 3.7V contains 74 Wh. A battery pack with 20,000 mAh at 3.85V contains 77 Wh.

Both have "20,000 mAh" on the label. The second has 4% more energy, but you cannot tell from the mAh spec alone.

Use Wh for comparisons. The Wh value is absolute — it does not depend on knowing the voltage. It is also the unit airlines use for carry-on compliance, so you need it anyway for travel.

Converting mAh to Wh

Formula: Wh = (mAh × V) / 1,000

Where V is the battery cell voltage. If the manufacturer does not state the voltage (many do not), use 3.7V as the standard lithium-ion nominal voltage.

Example: 26,800 mAh × 3.7V / 1,000 = 99.16 Wh

Better: look for the Wh printed on the bank itself. Required for airline compliance and usually printed on the label.

How Much Capacity Do You Need?

Rule of thumb: multiply the Wh capacity of everything you want to charge by 1.25 to account for conversion losses (typically 80% efficiency). That is your minimum Wh target.

Phone only (2 full charges): ~28 Wh → 10,000 mAh bank is sufficient Phone (5 full charges): ~60 Wh → 20,000 mAh bank Laptop (MacBook Air) + phone: ~80 Wh → 25,000 mAh / 90 Wh bank Laptop (MacBook Pro 16) + phone: ~120 Wh → 35,000 mAh bank (exceeds airline carry-on limit)

Output Wattage: The Spec Most People Miss

A power bank with 20,000 mAh and 18W output will not charge a MacBook Pro. The laptop requires at minimum 67W, and 18W is below the threshold where the laptop even begins charging.

Match output wattage to device requirements:

Device	Minimum W	Faster W
Phones (most)	18W	30W+
iPad Pro	20W	30W
MacBook Air 13"	30W	67W
MacBook Pro 14"	67W	96W
MacBook Pro 16"	96W	140W
Dell XPS 13	45W	65W
Gaming laptops	65W	100W+

Note: "minimum" means the laptop charges, but slowly. If you are using the laptop while charging, you may need the higher wattage to keep up with power consumption.

The Port Configuration Matters

Multi-port power banks split output wattage when multiple devices are connected. A bank rated at 100W total may deliver:

• 100W to a single USB-C device
• 65W to USB-C + 18W to USB-A = 83W total when two devices are connected
• 45W + 25W + 18W when three devices are connected

This is not deceptive — it is physics. Check the bank's documentation for the actual split configuration if you need to charge multiple high-wattage devices simultaneously.

Real Capacity vs. Rated Capacity

No power bank delivers 100% of its rated Wh to devices. Energy is lost to heat during the voltage conversion from 3.7V battery cells to the 5V/9V/20V output voltage.

Typical efficiency:

• Good quality banks (Anker, Baseus, Zendure): 80-87%
• Average quality banks: 70-78%
• Poor quality banks: 60-70%

A 100 Wh bank at 85% efficiency delivers 85 Wh to your devices. A 100 Wh bank at 65% efficiency delivers 65 Wh — 23% less, and you cannot tell from the marketing.

Independent reviews that measure actual vs rated capacity are the most reliable source for this information.

Airline Rules

The IATA standard that most airlines follow:

• Under 100 Wh: carry-on only, no approval needed (limit usually 2 per person)
• 100-160 Wh: carry-on only, airline approval usually granted on request
• Over 160 Wh: not allowed on commercial aircraft
• No power banks in checked luggage — this applies regardless of Wh

The 100 Wh limit is the practical constraint for most travel power banks. Banks advertised as "TSA approved" or "airline approved" typically mean they are under 100 Wh — not that they have any specific certification.

When purchasing for travel: verify the Wh printed on the bank is under 100 Wh. Do not rely on mAh-to-Wh calculations from the spec sheet if the printed Wh is unavailable.

Build Quality Indicators

A power bank is a lithium battery in a housing. Quality differences in the battery cells and protection circuits matter for both safety and longevity.

Signs of better quality:

• Wh rating printed on the unit
• UL, CE, or FCC certification marks
• Current-generation protection circuits (over-charge, over-discharge, short-circuit, temperature)
• Manufacturer warranty of 12+ months

Avoid banks that:

• Do not state Wh rating (red flag for airline compliance and capacity transparency)
• Have implausibly high mAh claims for their physical size (energy density has limits — physics)
• Cost significantly less than competitors at the same rated capacity